A Multiprotocol Label Switching (MPLS) network may provide Virtual Private Network (VPN) service for selected packets (traffic) transported between nodes, e.g., routers, over communication links of the network using label switched paths (LSPs). A Label Distribution Protocol (LDP) may be employed to establish the MPLS LSPs along a shortest path between a source router and a destination router as determined by an Interior Gateway Protocol (IGP) executing throughout the network. As a result, neighboring (peer) routers in the network may establish IGP as well as LDP adjacencies and logical peer connections (sessions) over the links for the purpose of exchanging/distributing routing information and abstracting the network topology.
Specifically, the peer routers along the shortest path may establish operational LDP sessions over the links to exchange label bindings needed to provide the VPN service. If an LDP session is not operational over a link along the shortest path, traffic may be dropped and the VPN service may fail. Such a situation may occur when an interface of a router coupled to the link becomes newly active, causing the link to come up and establishing an IGP adjacency that enables Internet Protocol (IP) forwarding of traffic over the link prior to either establishment of the LDP adjacency and session or distribution of all needed label bindings.
To reduce such VPN traffic drop, IGP and LDP may be synchronized to ensure that LDP converges (i.e., the LDP adjacency and session are fully established and that all label bindings are distributed) before IGP converges (i.e., the IGP adjacency and session are established and all routing information is distributed) over the link coupled to the newly active interface of the router. Request for Comments (RFC) 5443, entitled “LDP-IGP Synchronization”, dated March 2009, describes a technique that synchronizes IGP and LDP by discouraging use of the link for IP traffic forwarding as long as LDP has not converged. To that end, IGP executing on the router may advertise, e.g., via a link state advertisement (LSA) transmission or “flush”, a maximum cost metric for the link coupled to the newly active interface resulting in, e.g., a first LSA flush and a first shortest path first (SPF) routing computation. After LDP has converged and the labels have been distributed in the MPLS network, IGP at the router may then advertise a normal cost metric for the link resulting in a second LSA flush and a second SPF computation. Thus, the technique results in two rounds of LSAs and SPF computation for each newly active interface, which may lead to scalability and performance problems for the router when many interfaces are consecutively activated in a short period of time.
A second technique for implementing LDP-IGP synchronization delays establishment of (“holds down”) the IGP adjacency over the link associated with the newly active interface. While IGP adjacency is held down, LDP convergence may be completed over an alternate path that enables a router to reach a peer router of the shortest path. After LDP has converged and the label bindings have been distributed in the MPLS network via the alternate reachable path, IGP may advertise a normal cost metric for the link resulting in, e.g., a single LSA flush and a single SPF computation. However, this second technique requires existence of an alternate reachable path between the router and a peer router.